A separator for electrochemical batteries refers to an intermediate membrane that isolates a cathode and an anode from each other in a battery while maintaining ionic conductivity, thereby enabling charge/discharge of the battery.
Recently, along with a trend of pursuing light weight and miniaturization of electrochemical batteries to improve portability of electronic equipment, there is a need for high-power high-capacity batteries for electric vehicles. Thus, a separator for batteries is required to have reduced thickness and weight as well as excellent dimensional stability under heat and high tension so as to improve productivity of high-capacity batteries. In addition, recently, the separator is required to have not only permeability and thermal shrinkage resistance, but also properties for increasing lifespan of the batteries such as cycle characteristics, or properties for improving productivity of the batteries such as injection of an electrolyte.
In manufacture of batteries, a winding process takes a substantial time and must be performed at high speed in order to improve productivity of the batteries. In order to prevent process failure due to fracture of a separator during high speed winding, various studies have been made to develop a separator having high tensile strength.
As an example of well-known techniques for improving tensile strength of the separator, Korean Patent Publication No. 10-0943235 B discloses a method wherein a high-density polyethylene composition, a molecular weight of which is regulated at a specific high level, is used in manufacture of a base film for separators, thereby providing a separator having enhanced physical strength. However, this method has a limit in that components of a base film are restricted to specific materials, and also has a problem in that the method cannot be applied to various base films. Moreover, this method has a problem of deterioration in heat resistance due to increase in melt shrinkage of the separator despite increase in tensile strength.
Therefore, there is need for a method that can increase tensile strength of a separator based on physical approaches so as to be applied to various base films, instead of simply changing a chemical composition of a base film to increase tensile strength as in the related art, and for development of a separator having high tensile strength and low melt shrinkage to provide good heat resistance.